Almost all friendly matrices have many obstructions

Abstract

A symmetric $m\times m$ matrix $M$ with entries taken from $\{0,1,\ast\}$ gives rise to a graph partition problem, asking whether a graph can be partitioned into $m$ vertex sets matched to the rows (and corresponding columns) of $M$ such that, if $M_{ij}=1$, then any two vertices between the corresponding vertex sets are joined by an edge, and if $M_{ij}=0$ then any two vertices between the corresponding vertex sets are not joined by an edge. The entry $\ast$ places no restriction on the edges between the corresponding sets. This problem generalises graph colouring and graph homomorphism problems. A graph with no $M$-partition but such that every proper subgraph does have an $M$-partition is called a minimal obstruction. Feder, Hell and Xie have defined friendly matrices and shown that non-friendly matrices have infinitely many minimal obstructions. They showed through examples that friendly matrices can have finitely or infinitely many minimal obstructions and gave an example of a friendly matrix with an NP-hard partition problem. Here we show that almost all friendly matrices have infinitely many minimal obstructions and an NP-hard partition problem.